Scale-up routers use integrated hardware and software designs sold together by a single vendor. This tight integration between hardware and software provides good performance, but adding more capacity requires buying additional routers from the same vendor. This vendor lock-in effect increases costs for users.
Scale-out routers use off-the-shelf hardware and commodity software to provide routing functionality. Increasing capacity is cost-effective, but the loose integration between hardware and software adds substantial overhead, increasing the latency of scale-out routers compared to scale-up routers.